How? Telegrams from Kreskin?
Very large, and anomalously positioned radar contact inbound, ETA 30 minutes.
They've gotten exactly as much warning as they got OTL, which wasn't enough to have fighters ready. You've offered no reason, beyond your bald assertion, to show why anything has changed.
Except that OTL the formation was mistaken (through a miscommunication) for a flight of US aircraft that were due to arrive that morning from that direction. When no flight is expected and no aircraft are known to be out there a very large formation will raise eyebrows.
Guns with no crews or ammo don't defend well. You haven't explained why that state of affairs has changed one iota from OTL.
I have, you've just been too cussedly obstreperous to accept that the US officers will do anything other than sit around with the heads up their asses.